MNT_DETACH is missing from <sys/mount.h> in glibc-headers-2.3.2-101.4
on FC1 with some updated packages. This is surprising, since the
umount2 man page clearly mentions MNT_DETACH as a valid flag.
Defining MNT_DETACH to 2 (to match the kernel) in sys/mount.h makes it
compile, although this isn't the most desirable "fix" by far.
Oh, and I also needed to include <sys/mount.h> in main.c.